1. Field of the Invention
The present invention relates to a plane illumination apparatus and a backlight apparatus that use a light source for emitting coherent light beams.
2. Description of Related Art
As a backlight apparatus used in liquid crystal panels or the like, a system in which light is incident on an edge of a light guide plate, repeatedly reflected between two opposing surfaces by total reflection, and then taken out by a diffusion device or the like is known. This type of backlight apparatus includes backlight apparatuses using a cold cathode fluorescent lamp as a light source and also, recently, backlight apparatuses using an LED as a light source.
When a cold cathode fluorescent lamp is used, there is a problem in that it is difficult to make a backlight apparatus thin and power consumption increases. When an LED is used, although it is possible to make a backlight apparatus thin, since an LED is a uniform diffusion illumination device, it is difficult to make every light incident on a thin light guide plate with no leakage, hence loss is caused.
In contrast to above, a laser beam is excellent in straightness, and hence considered to improve light incidence efficiency.
However, when a laser beam is used as a light source, speckles due to high coherency of laser are generated. Speckles are a spotted pattern which is formed when a coherent light beam such as a laser beam is emitted to a scattering plane. If speckles are generated on a screen, they are observed as spotted luminance unevenness, i.e. brightness unevenness, thus becoming a factor of having physiologically adverse affect on an observer. It is considered that the reason why speckles are generated in the case of using coherent light beams is that coherent light beams reflected from respective portions of a scattering and reflecting plane such as a screen have very high coherency so that coherent light beams interfere with one another to generate speckles. For example, a theoretical review of the generation of speckles is made in detail in Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.
As discussed above, in the system using a coherent light source, since there is a problem of generation of speckles unique to the coherent light source, techniques for suppressing the generation of speckles have been proposed. For example, Japanese Patent Laid-Open No. 6-208089 discloses a technique in which a laser beam is emitted to a scattering plate, scattered light beams obtained therefrom are guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, thus reducing speckles.
SUMMARY OF THE INVENTION
Not only backlight apparatuses, speckles are problems in a variety of apparatuses having an illumination apparatus for illuminating an illumination zone with coherent light beams. Coherent light beams, for example laser beams as a typical example, show excellent straightness and can emit a light of extremely high energy density. Therefore, it is preferable for illumination apparatuses actually developed to design the optical path of coherent light beams in accordance with the characteristics of coherent light beams.
The inventors have extensively researched under consideration of the points discussed above, and as a result, the inventors have contrived the invention regarding a plane illumination apparatus and a backlight apparatus which illuminate a specific zone repeatedly with coherent light beams that are then diffused and taken out to the outside with speckles made inconspicuous. Moreover, the inventors have proceeded with researches and succeeded in improvement in the illumination apparatus to constantly prevent the generation of a region extremely bright in a specific zone illuminated with coherent light beams. Namely, the purpose of the present invention is to provide a plane illumination apparatus and a backlight apparatus that are capable of making speckles inconspicuous and effectively suppressing the generation of brightness unevenness in a specific zone.
In order to solve the problems above, according to an aspect of the present invention, there is provided a plane illumination apparatus comprising:
an optical device configured to be capable of diffusing coherent light beams from respective points to an entire region of the corresponding areas in a specific zone;
an irradiation unit configured to irradiate the coherent light beams to the optical device so that the coherent light beams scan a surface of the optical device; and
a light guide plate configured to make coherent light beams that are reflected at a surface of the optical device or that have passed through the optical device propagate and to take out the coherent light beams to outside,
wherein the irradiation unit makes the coherent light beams scan the surface of the optical device by changing propagation directions of the coherent light beams,
the light guide plate comprises a light take-out portion configured to take out coherent light beams to outside while making coherent light beams propagate between a first end face on which coherent light beams from the optical device are incident and a second end face that is provided to face the first end face, and
the specific zone is provided inside the light take-out portion or along the first end face, or along the second end face.
According to the present invention, it is possible to provide a plane illumination apparatus and a backlight apparatus that are capable of making speckles inconspicuous and effectively suppressing the generation of brightness unevenness in a specific zone.